Modern medical diagnostic methods, such as x-ray computed tomography (CT), can be used to obtain image data of a measurement object to be examined. The measurement objects examined are generally patients.
X-ray computed tomography—hereafter abbreviated to CT—is a specific x-ray imaging method, which differs in principle from conventional x-ray layer imaging methods in the structure of the image. With CT imaging systems transverse sectional images are obtained, in other words layers of the body are mapped essentially perpendicular to the axis of the body. The tissue-specific physical variable shown in the image is the distribution of the attenuation value of x-ray radiation μ(x,y) in the sectional plane. The CT image is obtained by reconstructing the one-dimensional projections of the two-dimensional distribution of μ(x,y) supplied by the measurement system used from very many different angles.
The projection data is determined from the intensity I of an x-ray beam after it has passed through the layer to be mapped and from its original intensity I0 at the x-ray source according to the absorption law       ln    ⁢                  I        o            I        =            ∫      L        ⁢                  μ        ⁡                  (                      x            ,            y                    )                    ⁢              ⅆ        l            
The integration path L represents the route of the x-ray beam in question through the two-dimensional attenuation distribution μ(x, y). An image projection is then composed from the measurement values of the line integrals through the object layer obtained with the x-ray beams in one direction.
Projections from the widest range of directions—characterized by the projection angle α—are obtained by use of a combined x-ray tube detector system, which rotates about the object in the plane of the layer. The most commonly used devices at present are what are known as “fan beam devices”, in which a tube and an array of detectors (a linear arrangement of detectors) rotate together in the plane of the layer about a center of rotation, which is also the center of the circular measurement field. “Parallel beam devices”, which have very long measuring times, are not described here. It should however be pointed out that it is possible to transform fans to parallel projections and vice versa, so the present invention, which will be described with reference to a fan beam device, can also be used without restriction for parallel beam devices.
Generally errors can occur in the measurement system of a CT unit. The measurement system includes the x-ray radiation source (tube), the tube-side diaphragms to collimate the x-ray beam fan and the detector. In the case of modern CT units the detector generally includes ceramic detector elements and a downstream electronics system for signal preparation (integration and amplification) and for analog-digital conversion. Errors with different causes in the measurement system can be discerned in the subsequently reconstructed CT image by interference from, for example, annular image artifacts, which from a diagnostic point of view alone have to be suppressed—in order to prevent misdiagnosis. It is therefore necessary to determine the error source, during service operations in particular.
Until now in the prior art it was necessary to record specific measurement data in additional scans, for example to exclude the detector as an error source, with the measured data being stored on data media and sent to the detector manufacturer for analysis (generally not electronically). There is as yet no method for verifying the functionality of the x-ray source. If a tube error is suspected, the tube has to be replaced, taken apart and examined mechanically.
Such a procedure is both time-consuming and cost-intensive.